Pearlizers are a long-established and proven medium for providing cosmetic products with an aesthetically attractive, rich and interesting appearance (Cold pearl surfactant-based blends, Crombie, R. L. International Journal of Cosmetic Science, 1997, 19: 205-214).
Typically, all cold-dispersible pearlescent concentrates are made of three components, a) pearly waxes b) surfactants and c) water. Suitable pearlisers or waxes are, for example, the monoesters and diesters of ethylene glycol, propylene glycol and oligomeric alkylene glycols of this type or glycerol with C16-22 fatty acids, fatty acids and also monoalkanolamides of fatty acids with C2 or C3 alkanolamines. Waxes are emulsified with surfactants in aqueous medium and then are gradually allowed to crystallize in the surfactant environment. This process results in a stable uniform dispersion of platelet like structures that reflect light.
The surfactants that are used to disperse the pearly waxes are from all categories, namely, anionic, cationic, amphoteric, zwitterionic and non-ionic (U.S. Pat. Nos. 5,560,873, 5,711,899, 6,147,124, & 7,578,995). Alkyl sulfates and alkyl ether sulfates are known for being harsh on both skin and hair. In fact sodium lauryl sulfate is taken as a standard irritant for irritancy measurement. Sodium lauryl ether sulfate (SLES) is made by sulphating ethoxylated lauryl alcohol of varying degree of ethylene oxide (0.5 to 3.0 moles of EO per one mole of fatty alcohol). Any grade of SLES that is available in the market contains significant amount of sodium lauryl sulfate (more than 20%) that is an established irritant to human skin and mucosa. In the class of anionic surfactants ‘sulfates’ in general are known to be the highest irritants compared to ‘sulphonates or sulphosuccinates’. While doing the cleansing job, alkyl ether sulfates strip away proteins and lipids of cuticle of hair and stratum corneum of skin. Skin's moisture regulation mechanism is seriously affected due to adverse action of harsh surfactants on the proteins and lipids of upper layers of stratum corneum.
Products ‘without sulfates’ have been launched that do less or no damage to hair. An example of this is L'Oreal's ‘sulfate-free’ ‘Everpure’ range of shampoos. Unilever's Dove range of body washes uses mild surfactants like cocoyl isethionate and sodium glycinate to reduce damaging effect of harsh surfactants like fatty alcohol ether sulfates (Nutrium technology, A novel glycinate-based body wash, K. P. Ananthapadmanabhan et al., Journal of Clinical Aesthetic Dermatology 23-30, 6(6), 2013). These rinse-off formulations being sulfate-free require sulfate-free pearlisers to improve the aesthetics.
In addition to the complex requirement profile which these formulations have to satisfy such as high brilliance, good particle fineness and high compatibility with other auxiliaries is the need of performance requirements relating to processability. Particularly, highly concentrated formulations intended for subsequent processing are expected to combine uniform, good physicochemical stability with low viscosity and flowability.
One of the problems involved in the production and use of pearlescent concentrates concerns their flowability and pumpability. In addition, the concentrates may even assume the form of mixtures which do not flow and also cannot be pumped by conventional equipment. Pearlescent concentrates that are commercially available undergo rheological changes during storage period resulting into low shelf life and serious difficulty in handling the non-flowable viscous material. This is reported in the technical data sheets of commercially available cold pearlescent concentrates, for example, Mackpearl series from Rhodia (Solvay) & Euperlan series from Cognis (BASF) wherein it is specified that outside the temperatures range of 15° C. and 35° C. the pumpability may be restricted due to an increase in the viscosity. It is also mentioned that during longer storage periods, slight separations might occur which can be eliminated by stirring.
Polyol fatty acid esters are often used as viscosity adjusters in the production of pearlizing concentrates. It was proposed in the U.S. Pat. No. 5,711,899 to reduce the viscosity of pearlescent concentrates by addition of small amount of low molecular weight polyhydric alcohols and thus to make them flowable and pumpable. The preferred polyhydric alcohol mentioned in this patent is glycerol. Table 2 mentioned in the patent shows a distinct reduction in the viscosity from 22,000 mPa-s to14,000 mPa-s on adding glycerol. Further, these pearlescent concentrates are stable for three months only.
German patent application DE 4103551 (Henkel) describes pearlizing concentrates in the form of flowable aqueous dispersions which contain 15 to 40% by weight pearlizing components, 5 to 55% by weight emulsifiers and 0.1 to 5 or 15 to 40% by weight polyols. Polyhydric alcohols are added in large amount to make them flowable, pumpable and obviating the need of preservatives.
Polyols are also used in European patent application EP 0205922 A2 (Henkel). This application relates to flowable pearlizing concentrates containing 5 to 15% by weight acylated polyglycols, 1 to 6% by weight fatty acid monoethanolamides and 1 to 5% by weight nonionic emulsifiers. According to the teaching of European patent EP 0569843 B1 (Hoechst), nonionic flowable pearlizing dispersions may also be obtained by preparing mixtures of 5 to 30% by weight acylated polyglycols and 0.1 to 20% by weight selected from nonionic surfactants. European patent application EP 0684302 A1 (Th. Goldschmidt) proposes the use of polyglycerol esters as crystallization aids for the production of pearlizing concentrates.
U.S. Pat. No. 7,268,107 (Cognis) describes highly concentrated pearlizing concentrates with a high content of pearlizing waxes and a low content of emulsifiers wherein 0.1 to 5% by weight polyol esters are added to control viscosity.
Patent application WO2004/028676 (Huntsman) talks about the problem of high viscosity, thickening on storage under warm conditions and stability in pearlescent concentrates containing aqueous mixture of amphoteric surfactants and acceptable glycol stearates. For optimum viscosity, the patent application describes use of non-ionic or anionic deflocculents. In addition, the pearlizing concentrates may contain non-ionic dispersants.
JP2008013581 discloses preparation of pearlescent concentrates with low viscosity by using non-ionic surfactants with an HLB value of 9˜12. Further, it is mentioned that non-ionic surfactant with an HLB of 15 or more cannot lower the viscosity to the desired flowable & pumpable level.
International patent application WO 94/24248 discloses pearlizing agents based on alkyl polyglycoside/betaine surfactants which also contain glycols.
U.S. Pat. No. 6,306,916 mentions that the pearlescent concentrates prepared by the teachings of U.S. Pat. No. 5,711,899 have pronounced non-Newtonian or thixotropic behavior. The viscosity of non-Newtonian fluids is dependent on shear rate and hence non-Newtonian behavior is unacceptable for pearlescent concentrates. To overcome the non-Newtonian behavior, U.S. Pat. No. 6,306,916 teaches addition of Alkyl polyglycosides to pearlescent concentrates.
Accordingly, there is a need for pearlescent concentrates which have low viscosities and do not exhibit an increase in viscosity during storage and which are suitable for sulfate-free formulations.
The present invention addresses the problem of prior art by providing sulfate-free pearlizing concentrates which prevent viscosity-build up, without addition of polymers or inorganic carriers.